Drew
We did a commparrison cook on the Q120, Q220, Q300, Spirit 310 and Genesis 320. We cooked a porterhouse steak on each, we preheated them all for 10 minutes, the steaks came off the same slab and were the same thickness, we prepared each steak the same brushed with oil and seasoned one side. The steaks were put on together and turned after 2 1/2 minutes and cooked for a further 2 1/2 minutes. There was no distict differences with the steaks, however we did agree that the stek cooked on the Genesis was marginally better. The Q300 had slightly better grill marks. All were excellent, very moist and so tender you could cut them with a bread and butter knife.
I cook nearly everything on my Q including upside down cakes, muffins, quiche curries and stews. I know this is not going to help but you can do exactly the same on the Spirit - you just need to know how to set it up for indirect over a larger area of the grill. The parrallel sides enables you to put more and bigger joints of meat on the Spirit. You can use a rotis with the Spirit. (I do not use a rotis even on my Genisis).
It is easier to add smoke to a Spirit you just put one or two chunks of wet wood over the interconnect tube between the flavourizer bars.\
My preference is still the Q220.
I should add that I have a Genisis 320, Q220 (I just upgraded from a Q200 by replacing the lid) a Performer, 2 OTS kettles, a home made hot smoker and use a demonstration model of the WSM.
Again, I have probably made the decision harder for you, you need to list what you ideally want and check them off against each BBQ. Thats what I do with customers when I am demonstrating, I generally take the price out of the considerations and then look at what features the customer wants, the available room, the style of cooking that they want to do and the amount of people that they will generally cook for.
Hope this helps
Regards